This invention relates to a polymer electrolyte membrane suitable for use in polymer electrolyte fuel cells. More particularly, the invention relates to a polymer electrolyte membrane in polymer electrolyte fuel cells that has improved oxidation resistance and good adhesion to electrodes.
Polymer electrolyte fuel cells feature high energy density, so they have potential use in a wide range of applications including power supplies to household cogeneration systems, power supplies to mobile communication devices, power supplies to electric cars, and convenient auxiliary power supplies.
In a polymer electrolyte fuel cell, the polymer electrolyte membrane functions as an electrolyte for conducting protons and it also plays the part of diaphragm which prevents direct mixing of the fuel hydrogen or methanol with oxygen. The polymer electrolyte membrane which plays the part of an electrolyte causes an electric current to flow over a prolonged period, so it has several requirements to meet: good electrochemical stability, in particular, good stability in acidic aqueous solution (acid resistance), good resistance to peroxide radicals or the like (oxidation resistance) and good heat resistance, as well as high ion conductivity. In addition, the polymer electrolyte membrane which also plays the part of diaphragm is required to have low permeability to the fuel hydrogen gas or methanol and oxygen gas, as well as having high membrane's mechanical strength.
A common example of such polymer electrolyte membrane has been Nafion® which is a perfluorosulfonic acid-based membrane developed by DuPont. The conventional fluorocarbon polymer ion-exchange membranes such as Nafion® have outstanding chemical stability but, on the other hand, they have several problems including low electrical conductivity, insufficient water retention which causes the ion-exchange membrane to dry up, which in turn leads to a further decrease in electrical conductivity, and in the case of using methanol for fuel, the membrane swells in alcohols and cross-over of the methanol lowers the characteristics of the fuel cell. If, in order to deal with these problems, one introduces more sulfonic acid groups, the membrane strength drops markedly upon holding water, whereby it will break easily; hence, it has been difficult to meet both requirements for electrical conductivity and membrane strength. Still another problem of the fluorocarbon polymer electrolyte membranes such as Nafion® is that the synthesis of fluorine-containing monomers as a starting material is complex enough to make the product membrane very expensive, and this presents a large obstacle to realizing commercially feasible polymer electrolyte fuel cells.
Hence, efforts have been made to develop low-cost and high-performance polymer electrolyte membranes that can be substituted for Nafion® and other conventional fluorocarbon polymer electrolyte membranes, and an example that has been proposed is a polymer electrolyte membrane that is synthesized by first introducing through a radiation-induced graft reaction a styrene monomer into an ethylene-tetrafluoroethylene copolymer (ETFE) film having a hydrocarbon structure and then sulfonating the introduced monomer (see, for example, JP 9-102322 A). However, this polymer electrolyte membrane has a serious drawback; the main chain of the polymer membrane and the polystyrene graft chains are composed of hydrocarbons, so if a large electric current is applied to the membrane for a prolonged period, both the hydrocarbon chain portion and the polystyrene graft chain portion undergo oxidative deterioration and the electrical conductivity of the membrane drops considerably.
In another example, it has been proposed that the cell characteristics be improved by forming graft side chains of a crosslinked copolymer using styrene and divinylbenzene, such that the graft chains have a crosslinked structure introduced thereinto (see, for example, JP 11-111310 A). This method of crosslinking the graft chains with divinylbenzene offers the advantage of improving oxidation resistance by allowing a greater amount of divinylbenzene to be introduced; on the other hand, the membrane becomes less flexible, probably because divinylbenzene is localized on the membrane surface during graft polymerization.
In general, as the electrical conductivity of a polymer electrolyte membrane increases, the internal resistance of the cell decreases and it outputs more power. However, some of the conventional polymer electrolyte membranes output only low power even if their electrical conductivity is high. One reason for this problem is that due to prolonged use, the adhesion between either electrode and the polymer electrolyte membrane decreases and a gap forms at the interface to reduce the electrical conductivity in that area. In addition, if the polymer electrolyte membrane becomes less flexible, its adhesion to either electrode decreases, again reducing the electrical conductivity in that area.
A need therefore exists for developing a polymer electrolyte membrane for polymer electrolyte fuel cells that has improved oxidation resistance and good adhesion to electrodes.